Although other cycles have been identified and used in absorption refrigeration heat pump and refrigeration equipment applications, ammonia-water (AW) absorption cycles remain an attractive approach to gas-fired, compact, air-cooled, air conditioning, heat pumping, and refrigeration equipment. However, for these cycles to continue to be accepted by the marketplace, long equipment lifetimes and low maintenance are essential. To do this, corrosion must be kept to a minimum. Present systems use a chromate corrosion inhibitor that many consider to be hazardous.
Carbon steel is the material of choice for constructing ammonia-water refrigeration machines. The use of inexpensive materials such as carbon steel is necessary because of the cost sensitive nature of the ammonia-water market. However, steels are susceptible to corrosion in the highly alkaline environment. Corrosion of steel not only shortens the life of a machine but also reduces its efficiency due to buildup of hydrogen gas that is generated by corrosion processes. The accepted practice to minimize corrosion in this system is by adding a corrosion inhibitor to the refrigerant.
The most widely used inhibitor in commercially available AW systems is sodium chromate, which has been found to provide adequate protection against corrosion for this application. However, chromate-base compounds are toxic and some forms are known to be carcinogenic. In recent years there has been increasing restrictions put on the use of chromate-based compounds by regulatory agencies because of health and environmental concerns. Therefore, there is an urgency to find an effective and environmentally acceptable non-chromatic inhibitor for refrigeration machines.
Ammonia-water (AW) refrigeration machines require an inhibitor to protect the internal wetted steel parts from corrosion. The inhibitor most widely used by the heating, ventilation, and air conditioning (HVAC) industry is sodium chromate. However, because of the toxicity of hexavalent chromium in the chromate, the inhibitor is considered an environmental pollutant, and is likely to be prohibited for use in many localities. Alternative inhibitors are needed or required to replace the sodium chromate in the AW machines.
It has been reported that the presence of silicate ions improves the tensile strength of Type 403 stainless steel in aqueous solutions at pH 7 to 12.2 at 212.degree. F. (100.degree. C.). However, it is also well known that the presence of ammonia can make a significant difference in solution chemistry. It can be shown that many inhibitors that function in aqueous systems (water only) will not function once the ammonia exceeds a critical concentration.
Corrosion and inhibition in aqueous ammonia systems will be very important to the future HVAC industry if anyone of a variety of aqueous ammonia gas-fired air conditioners or heat pumps reach the marketplace. Therefore, the distinction of using valuable alkali salts in ammonia water cycles is very important.
For the most part, the prior art reveals the use of alkali salt silicates in so-called "antifreeze" compositions to inhibit corrosion in the radiator's components of heat transfer machines, particularly automobiles. In the case of ammonia water machines simultaneous heat and mass transfer occur, whereas in automobile radiators and other heat exchangers only heat transfer takes place. Therefore, the prior art has not revealed the effects of inhibitors on components exposed to simultaneous heat and mass transfer operations. Examples in the prior art of prior usage are as follows:
U.S. Pat. No. 4,961,878 Mullins discloses a composition for inhibiting metal corrosion in a closed aqueous cooling system comprising an aqueous mixture of a nitrate, a silicate, an acrylate polymer, an amine oxide and tolyltriazole. The silicate is introduced as a sodium salt.
U.S. Pat. No. 4,915,872 Ciuba et al. shows a solid cast block of corrosion inhibitor comprising about 5 to 20 wt % silicate, calculated as sodium metasilicate pentahydrate, about 5 to 90 wt % borate, calculated as sodium tetraborate pentahydrate, about 0.5 to 5 wt % polymeric dispersant such as polyacrylamide, and a sufficient solidifying amount of water.
U.S. Pat. No. 4,098,720 Hwa reveals low toxicity, low pollution potential compositions and methods for inhibiting corrosion of ferrous and non-ferrous metallic parts in aqueous systems. The compositions include an alkali metal silicate and small amounts of alkali metal hydroxide.
U.S. Pat. No. 4,724,125 Tsuneki et al. discloses metal materials in contact with aqueous systems are prevented from corrosion by a method which comprises adding copolymer having a molecular weight in range of 1,000 to 20,000 and formed between isobutylene and at least one member selected from among maleic acid, water-soluble salts thereof, and maleic anhydride of water of such quality that the Langelier index is not less than 1.5 and the relation (SiO.sub.2).times.(CaH) is greater than or equal to 2000 where SiO.sub.2 stands for the SiO.sub.2 concentration in the water (mg/liter) and (CaH) for the calcium hardness (mg/liter as CaCO.sub.3) in water.
U.S. Pat. No. 4,382,008 Boreland et al. reveals corrosion inhibitors for aqueous media comprising a triazole, an alkali metal borate, benzoate, silicate and an alkali metal salt of C.sub.7 to C.sub.13 dibasic acid. These inhibitors may be used in antifreeze compositions.
U.S. Pat. No. 3,215,637 Clerbois shows a process for the protection of metals against the corrosive action of brines which contains a mixture of sodium silicate and zinc chloride in selected proportions to inhibit corrosion by sodium chloride and calcium chloride brines.
U.S. Pat. No. 2,972,581 Johnson et al. reveals a corrosion inhibitor and cooling solution consisting of mercaptobenzothizole, alkali metal salts, an alkali metal silicate, and alkali metal nitrite, and an alkali metal borate.
The various prior art disclosures do not reveal the use of silicon compounds that are useful in ammonia-water absorption cycles and in association with ammonia and water solutions.